The invention relates to an electrode clamping device suitable for use in an electrical arc furnace.
Arc furnaces are frequently used in the steel and ferro alloy production industry during metallurgical smelting operations. An electric arc furnace comprises one or more electrodes that extend into a furnace. Lower ends of the electrodes are located adjacent a furnace load, and in use supplies the required energy to melt the load by forming an electric arc between the electrode and the furnace load. The electric current required to achieve the “arcing” is conducted to the electrode by way of conductive contact shoes, which provides a conductive path between the energy source and the electrodes.
Arc furnaces also include positioning systems which are designed to hold the electrodes, and also to control the position of the ends of the electrodes relative to the load so as to ensure that approximately constant current and power input are maintained during the melting or smelting of the load. The various positioning systems typically have the same common denominator of having a yoke that releasably engages the electrode, with the yoke being displaceable relative to the roof of the furnace so as to control the position of the electrode. The yoke is generally displaced by a winch or a hydraulic piston arrangement.
During operation the electrodes are consumed at lower ends thereof, and needs to be continuously displaced downwardly to ensure that terminal ends thereof remain proximate the furnace load. To some extent, the positioning system as described above is used to control the position of the end of the electrode. However, at a certain point the positioning system will reach an absolute lower limit, and the electrode will have to be readjusted relative to the electrode holder in order to allow further downward displacement of the electrode. In practice, this means the electrode must be allowed to be downwardly displaced relative to the electrode holder, and this process is generally referred to as electrode slipping. In some cases, for example where excessive slip has been allowed, there may be a need to displace the electrode upwardly relative to the electrode holder. This process is referred to as back slipping.
In most existing arc furnaces slipping is enabled by providing a set of two vertically adjacent clamping devices. The first clamping device is provided on the yoke, and the second clamping device is spaced from the first by means of hydraulic pistons. When slipping is required, one of the two clamping devices is released and moved away from the other that is still holding the electrode when in the desired position it reengages the electrode, at this point the other clamping device is released and the two is then moved closer to each other “slipping” the electrode downwardly, once the desired “slip” is reached both clamps may be reengaged to hold the electrode. It will be appreciated that there may be many different configurations through which the above methodology can be implemented. However, all the configurations share the common denominator of having two clamping devices, each of which is required to exert a clamping force on the electrode in a first, clamping condition, and in most cases to exert no clamping force or a reduced clamping force on the electrode in a second, release condition.
The slipping process has to be done in a very controlled manner due to the size, weight and sensitivity of the electrodes to breakages. In addition, an outer surface of an electrode generally has a relatively low coefficient of friction, which renders the proper clamping of an electrode, especially during slipping, critical. In smelters that utilize Soderberg electrodes of the smooth type the clamping is typically done at a level where the thin steel electrode casing or shell is the only source of structural support, and the clamping must therefore be done in manner that will not result in crushing of the thin casing or shell. In order to achieve this it is imperative for forces to be distributed evenly right around the electrode.
On large electrodes over about 800 mm in diameter, a number of different clamping device designs are known in industry, and from a functional design perspective they can generally be divided into two major groups. A first group of clamping device are all characterised in that the clamping force is applied in a radial direction at a number of discrete points of clamping right around the electrode. In a second group of clamping devices, the clamping force is generated circumferential about the entire periphery of the electrode like a wire hose clamp.
As mentioned above, in the first group the clamping force is applied radially, for example by arranging sets of springs around the electrode. The sets of springs then apply direct radial or near radial pressure on the electrode, typically from four or more sides. In this design the required clamping force is quite high, and is determined only by the mass of the electrode and the achieved friction coefficient between the holding shoes and the casing. For example, for an electrode of 40 ton having a friction coefficient of about 0.4, the required radial force would be about 100 ton, which must be divided between the number of clamping members if four segments then that will be 25 ton each. The reaction forces are taken up in a frame that surrounds the electrode and houses the force generating devices. This kind of device then also needs multiple de-clamping devices to remove the applied force, so if the force is applied from four directions then four de-clamping devices are also required because each force generating mechanism is a functionally discrete unit.
A first disadvantage of this type of clamping device is that where springs are used for the force generating device, the springs need to be preloaded by compressing the springs with a suitable adjustment mechanism. Considering that in the above example (four clamping points) the forces are applied in 90 degree segments then one would have to preload a 25 ton set of springs at each clamping point. This is not an easy task and can cause significant delays during setup and maintenance. A further disadvantage is that this kind of design is also very heavy, as it needs a structural frame, multiple de-clamping devices, and large and very heavy springs. The spring mechanism can also be very expensive and difficult to obtain, for example if cup springs are used which also have other disadvantages.
The second group of clamping device, as already mentioned above, is the family of clamping devices where an actuation force is distributed in a circumferential manner, but the corresponding clamping force is then exerted on the electrode in a radial direction. This is therefore effectively an arrangement where a clamping ‘band’ extending about the circumference of the electrode is tensioned. The term ‘band’ is of course used loosely, and should be interpreted to include a cable, chain, a plurality of linked elements, or any suitable elongate tensioning element that can be positioned about the periphery of the electrode, and which can transfer a tensile load.
The clamping force is applied by pulling at the ends of the band(s), and the direction of the applied tensioning force is therefore in all cases essentially tangential relative to the electrode. This force distribution about the periphery of the electrode results in a considerable reduction in the required actuation force, and for the same electrode mentioned in the example above the required actuation force reduces from 100 ton to about 20 ton. As a further advantage this kind typically only needs a single de-clamping device as only a single actuation force-generating device can be used. However this kind of clamping device needs some additional equipment to ensure that the circumferential force is distributed around the electrode. This can be by means of levers, hinges, flexible bands, linkages or cables. To achieve a symmetrical design the force needs to be applied through lever arms of some sort, which makes the force-generating device quite large, heavy and expensive. The lever arms also introduce additional maintenance requirements.
The use of the lever arms furthermore increases the required travel of the de-clamping device during de-clamping of the clamping device. Far example, in some cases a spring travel of 90 mm is required in order for the band to be slackened by 30 mm (lever arms of 3:1 ratio) which then gives less than 5 mm radial release on the electrode. This is not ideal, as the required spring displacement should be kept to a minimum. If no symmetry is needed the lever arms will not be required, but in such configuration the force-generating device protrudes quite far from the electrode, which may not be acceptable from a practical perspective.
An advantage of this type of clamping device is that typically no structural frame is needed for the force-generating device to act against. Furthermore, during setup and maintenance the de-clamping device can be used to compress the force-generating devices (springs) further, and adjustment is therefore done without needing to pre-stress the springs manually.
It is accordingly an object of the invention to provide an electrode clamping device that will at least partially, alleviate the above disadvantages.
It is also an object of the invention to provide an electrode clamping device which will be a useful alternative to existing electrode clamping devices.
It is a still further object of the invention to provide a clamping device suitable for use in electrode clamping and slipping assembly.